An artificial solar pond is a man-made body of standing water designed to collect solar radiation and store heat over a relatively long period of time. It has a multilayer regime: a wind-mixed layer at the surface of the pond, a halocline below the wind-mixed layer, a heat storage layer below the halocline, and a thermocline at the bottom of the pond. The wind-mixed layer has a depth from 10-50 cm depending on weather conditions and has a uniform salinity from 3-5%. The halocline is from 1-1.5 m deep and has a salinity profile that increases monotonically with depth to a salt concentration of about 25-30%. Usually the heat storage layer is of uniform salinity and has a depth adequate to provide the amount of heat storage required. Finally, the thermocline is a stratified layer usually of uniform salinity at the bottom of the pond.
The wind-mixed layer is convective and its temperature approximates ambient temperature. The halocline is non-convective by reason of its density gradient with the result that solar radiation absorbed within the halocline heats the halocline and sets up a temperature gradient matching the density gradient. Heat from the halocline is transfered to the underlying heat storage layer by conduction across the interface between the halocline and the heat storage layer. The halocline thus acts as an active insulator for the heat storage layer preventing cooling by conductive heat transfer to the ambient atmosphere above the wind-mixed layer. The heat storage layer is convective by reason of the mixing of the layer due to the transfer of heat from the heat storage layer for useful purposes, and will be of a substantially uniform temperature. The lowermost layer in the pond is the thermocline which is stratified and serves as an insulator against conductive cooling of the heat storage layer to the ground below the pond.
Heat can be extracted from the heat storage layer by placing a heat exchanger within the heat storage layer and pumping a heat exchange fluid through the heat exchanger. Usually, the heat exchange fluid will be an organic working fluid such as Freon which is vaporized in the heat exchanger and then expanded in a turbine producing work. The exhausted vapor from the turbine is then condensed and returned to the heat exchanger which acts as a boiler for the turbine. Alternatively, heated water from the heat storage layer can be pumped to a heat exchanger external to the pond and then returned to the heat storage layer.
At latitudes corresponding to Southern California, Arizona, and New Mexico in the United States, the average solar input, the year round, day and night, is about 250 Watts/m.sup.2. A solar pond is about 20% effecient in converting this energy into heat with the result that the average heat input to the heat storage layer of a pond situated at the indicated latitude is about 50 Watts/m.sup.2. On an annual basis, the solar power input to the heat storage is about 400 KWhr/m.sup.2. Thus, in one year, a one acre pond will store the heat equivalent of about 1.6 mKWhr. If the water temperature of the heat storage layer is about 80 degrees Celsius, the heat storage layer will have to be about 2 meters deep.
When a solar pond is located in an arid environment where fresh water is scarce, water to construct the pond or to compensate for evaporation losses after the pond is in use may be made available by constructing catch basins to collect run-off water that normally would be dissipated. Such run-off water originates from limited rainfall in the area of the solar pond, or from the discharge of irrigation pipelines. As a consequence of flow over the ground and into the catch basins, the run-off water is contaminated with nutrients such as phosphates and nitrates. Using water so contaminated to construct a pond will present a favorable environment for the growth of photosynthetic microbes that exhibit halophilism, i.e., microbes that demand salt for growth and maintenance. The level of salt present in a given layer in the pond will determine the type of halobacteria that can grow. For example, halophilic algae, molds, and amebae will grow in the moderately saline environment of the wind-mixed layer and in that portion of the halocline in a solar pond which does not exceed 50.degree. C. Particularly when the water in the pond is rich in nutrients, the growth of Dunaliella viridis, Dunaliella salina, and other Dunaliella spp. will be encouraged. Blooms of these halophilic materials in the upper portions of the pond causes turbidity which will reduce light transmission into the halocline and thus decrease the ability of the pond to absorb useful solar radiation that can be transfered to the heat storage layer.
In principle, blooms of algae can be controlled by the addition to the pond of herbicides or poisonous metallic salts such as copper sulphate. This conventional approach to controlling algae growth, however, requires periodic checking of the concentration of the anti-algae materials in the pond. Another disadvantage of the conventional approach to controlling algae lies in the environmental impact of poisonous materials in a large body of water.
It is, therefore, an object of the present invention to provide a new and improved method of and apparatus for controlling the turbidity in a body of brackish water without the use of herbicides or poisonous salts.